Consistent explanation for the cosmic-ray positron excess in p-wave Sommerfeld-enhanced dark matter annihilation

Abstract

Sommerfeld-enhanced dark matter (DM) annihilation through $s$-wave has been widely considered as a consistent explanation for both the observed cosmic-ray (CR) positron excess and the DM thermal relic density. However, as the $s$-wave Sommerfeld-enhanced annihilation cross section increases monotonically with decreasing DM velocity, severe constraints appear from the data of gamma rays from dwarf spheroidal satellite galaxies (dSphs) and the cosmic microwave background (CMB), as the relevant typical DM velocities are even lower than that in the Galactic halo. In this work, we consider Sommerfeld-enhanced $p$-wave DM annihilation where the DM annihilation cross section can be enhanced at certain velocities but eventually be highly suppressed when the DM velocities become extremely low. We calculate the velocity-dependent astrophysics factors ($J$-factors) for the Sommerfeld-enhanced $p$-wave DM annihilation for fifteen nearby dSphs. Taking the channel of DM annihilating into $4\mu$ through two light mediators as an example, we show that there are parameter regions where this mechanism can account for the CR positron excess and the DM relic density, while being compatible with the constraints from dSphs gamma rays measured by Fermi-LAT and that from the CMB measured by PLANCK.